CHAPTER III 



THE SIMPLEST ANIMALS — PROTOZOA 



24. Definitions. — In our study of the grasshopper and 

 its insect relatives we considered their behavior and life 

 processes. If we had studied the minute structure of any 

 of these insects, the grasshopper, for example, and had 

 used a microscope to aid us, we should have found that 

 every organ was made up of numerous small parts joined 

 together in a definite manner. These small parts are 

 called cells. 



Any book on biology uses the word cell again and again. 

 The name was first used by Robert Hooke over two hun- 

 dred years ago, when, with his crude microscope, he 

 examined a piece of bark and found it to be made up of 

 little rooms which looked like the cells of the honey com I). 

 These spaces he named cells. When better microscopes 

 were made, the living parts of the cell were discovered, 

 and it was found that Hooke had seen only the walls of 

 dead cells. 



All plants and animals are composed of cells. A cell 

 may exist alone, carrying on all the life processes itself, 

 or it may exist in connection with a great many other 

 cells, as in all large animals and plants. In every case 

 each cell is produced from another cell. 



There are certain animals that are never more than one- 

 celled even when they are full grown. These animals are 

 called Protozoa (pro-to-zd'a: Greek, protos, first; zoon, 

 animal). 



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